1. Field of the Invention
This invention is directed to a device and method for removing fruit replies from a reply data signal to produce a defruited data signal which is provided to a data processor in a radar system for further processing.
2. Description of Related Art
Radar systems commonly use an interrogator/receiver and antenna system for soliciting information from aircraft passing within range of the radar system. The interrogator/receiver and antenna system typically sends an interrogation signal outward at each range and azimuth interval within the radar system's coverage area. Aircraft present at a given range and azimuth interval respond to the interrogation signal by sending a reply signal carrying the solicited information to the interrogator/receiver and antenna system. However, it occasionally happens that a different aircraft responding to a different interrogator/receiver and antenna system sends an unsolicited reply signal to the subject interrogator/receiver and antenna system. This false reply signal is termed `fruit` (false reply unsynchronized in time). In order for a radar system to operate properly, provision must be made to prevent the radar system from interpreting the fruit reply as a true reply. In an environment in which the number of fruit replies received by a radar system is large, an excessive amount of computing time is absorbed by a data processor in eliminating fruit replies.
Radar systems commonly operate in one of two modes. `Beacon` mode is primarily used by commercial aircraft while `mode-4` is commonly used by military aircraft. In beacon mode, reply data is processed to determine whether no reply or a true reply occurred in a given range interval for a given pulse repetition time (PRT). A pulse repetition time is the time required by the radar system to complete one sweep of the coverage area of the radar system. In contrast, mode-4 reply data is processed to determine whether no reply, a near miss or a true reply occurred in a given range interval for a given PRT. A near miss reply is defined as the absence of a reply at the same range as a true reply occurring within an interval of PRTs after a true reply. The purpose of detecting near miss replies for mode-4 processing is to provide information on the spoof environment in the vicinity of true replies to determine if the target is a friend or foe. Consequently, mode-4 operation requires that near miss replies be processed in addition to no and true replies. In either mode-4 or beacon mode, replies must be evaluated to determine whether such replies are fruit.
One technique for reducing the number of fruit replies received by a data processor of a radar system operating in beacon mode, and therefore the processing time expended by the data processor in beacon mode, is disclosed in the U.S. Pat. No. 4,896,158 (the '158 filter). Referring to FIG. 1, the '158 filter includes an age window which stores reply data corresponding to a predetermined number of pulse repetition times (PRTs). A pulse repetition time is the time required by the radar system to complete one sweep of the coverage area of the radar system. The age window of the '158 filter is further divided into a hold window which includes reply data for a smaller number of PRTs than that of the age window. The '158 filter observes the hold window and determines the number of related replies occurring within each of the PRTs included within the hold window. When the number of related replies within the hold window is below a threshold value, the '158 filter eliminates those replies as fruit replies. On the other hand, when the number of related replies included within the hold window is greater than or equal to the threshold value, the '158 filter then observes the age window to determine the total number of related replies included within the age window. When the total number of related replies included within the age window is below a second threshold value, the total related replies are discarded by the '158 filter as fruit. Conversely, if the total number of related replies within the age window is greater than or equal to the second threshold value, the reply data corresponding to the total number of related replies is provided to the data processor of the radar system for further processing.
The efficiency of the '158 filter is decreased due to the fact that, in general, the reply data provided to the data processor of the radar system is not received in the sequential PRT in which the reply data was received by the radar system. This problem occurs because the reply data pertaining to a particular number of related replies is provided to the data processor whenever the particular number of related replies exceeds the first and second threshold values for the hold and age window, respectively. For purposes of illustration of the problem, assume that in FIG. 1 the first threshold value for the hold window is 2, that the second threshold value for the age window is 4, that a first set of related replies occur in PRTs A, B, F and G, and that a second set of related replies occur in PRTs A, B, I and J. Both sets of related replies will be considered true replies by the '158 filter. However, they are provided to the data processor simultaneously even though the first reply in each of the two sets was received at different PRTs by the radar system. This problem requires the data processor to expend computing time in correlating replies with the actual PRT in which they occurred. Thus, there is a need in the art for a filter which eliminates the computing time needed to arrange replies which are provided to the data processor of a radar system in the sequential order in which such replies are received by the radar system. Further, there is a need in the art for a filter which operates in mode-4 since the '158 filter does not provide a fruit filter with near miss processing capability.